Homopolyatomic Cations of the Post-Transition Elements

Naked polyatomic clusters of post-transition metals can also be obtained as cations. They correspond to intermediate species between the metals and the compounds in conventional oxidation states. The well known dimercury cation stable in acidic conditions, may be considered a prototype of this kind of species. The best known examples of this class of compounds are the proper cluster species Bi and Big (vide infra). 

The limiting process for the stability of polycations is the disproportion reaction to the metal and compounds in normal oxidation states, for instance, 

On the other hand, the stability of polycations also requires the absence of oxidizable species that might induce the reduction to the metal. 

The reaction conditions required for the preparation of polycationic ions are thus essentially opposed to those used for Zintl ions discussed above. For the stabilization of these species, extremely poor electron donor media are needed. Even the less basic usual molecular solvents often turn out to be not inert enough toward polyatomic cations. They can be better stabilized in strong 

The requirements of the counterions are also extreme. Large anions with relatively high inertness toward oxidation and with low basicity through coordination or dissociation to more basic fragments are the more appropriate for stabilizing naked cationic clusters. Best results have been obtained with the tetrachloroaluminate anion [AICI4] which is normaly generated in situ from the metal halides and aluminum chloride used as solvents. 

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